If you have a bottle of sunscreen left over from last summer that's already been opened, you shouldn't use it.
Better to throw it out and buy a new one instead, dermatologists say.
There are two major reasons not to use sunscreen that is too old.
The first is that it likely won't even do what it's supposed to, namely, to prevent you from getting sunburnt.
Having been exposed to oxygen, the sunscreen's contents will have changed under oxidation and can no longer properly fulfil their protective function, explains Dr Eckhard Breitbart, chairman of the Hamburg-based Dermatological Prevention Working Group (ADP) and a German Cancer Aid (DK) expert.
But that's not all.
"Substances can form that may well be carcinogenic," he says, though he adds there's no "solid proof" of this as yet.
However, researchers did find in 2021 that the organic compound benzophenone, which has been identified as a possible human carcinogen, accumulates over time from the degradation of the ultraviolet light (UV) filter octocrylene used in commercial sunscreen products.
The maximum amount of time you should use a sunscreen after opening it is marked on the container with the symbol of an opened jar showing the limit, e.g. "12M", or 12 months.
So as not to forget when you opened it, it's a good idea to write the date on the container with a permanent marker.
Some sunscreens also have a best-by date.
If it's expired, the product shouldn't be used even if it's still unopened, Dr Breitbart advises.
A best-by date is often lacking on cosmetic products sold around the world, however.
The shelf life of sun protection products, whether creams or sprays, is at least 30 months from the date of manufacture, so long as they're keep in a cool, dry place, according to the German personal care brand Nivea.
Discard your old sunscreen if it has a pungent or unusual odour, has become discoloured, or if the consistency has changed by becoming runny or grainy, for example.
With regard to the choice of sunscreen, Dr Breitbart advises to go for perfume- and fragrance-free products, and to make sure that they protect against UVA and UVB rays.
When applying sunscreen, be generous, as it will only give you the protection factor it says on the tube if you actually put enough on.
"A rule of thumb is: For adolescents, a heaped teaspoon is needed on the face alone - for adults a little more," he says.
It doesn't pay to be sparing in your use of sunscreen, he says, because its stated SPF (sun protection factor) can only be met if the sunscreen is applied liberally to the skin.
"A rule of thumb is that adolescents need a heaped teaspoonful for their face alone, and adults somewhat more."
Dr Breitbart also notes that sunscreen alone isn't enough to keep your skin cancer risk low.
Wearing sun protection clothing is important too, as is avoiding direct sun exposure when UV radiation is strongest, e.g. at midday.
Clothing provides widely varied UV protection.
Dark blue jeans have an SPF of about 500, compared with just 10 for a thin, white cotton T-shirt.
There's specially-designed sun protective clothing whose fabric is rated for its level of UV protection.
Frequent washing lowers the level, however, and people who go bathing should bear in mind that wet fabric is less UV protective.
A common misconception, DK says, is that protection from UV radiation isn't necessary on cloudy days.
In fact, clouds affect UV radiation in different ways.
While thunderclouds greatly weaken it, other types of clouds and cloud combinations can actually intensify it due to scattering effects.
Generally speaking, clouds weaken UV radiation intensity by only 10% to 50%, according to DK, pointing out they don't offer reliable protection particularly at midday.
A suntan isn't very protective either, contrary to what many people think.
Aside from the fact that it's a stress response by the skin, a suntan provides the equivalent of merely about SPF 4, less than any sunscreen.
This risk is especially high on beaches with a light breeze, where people often underestimate the amount of UV radiation – though it's impossible to perceive with the senses anyway.
Dr Breitbart recommends checking the UV index forecast provided by national weather services.
The index scale ranges from 1 to 11.
Once the value reaches 3, you should take measures to protect your skin, from putting on clothing to applying sun cream.
People who exercise outdoors are particularly exposed to UV radiation.
They are best advised to work out in the early hours of the morning or later in the evening, and should apply sunscreen 20 to 30 minutes beforehand, says Matthias Schoenmann-Finck, a lecturer at the German University for Prevention and Health Management.
It's important to choose a waterproof product that sweat can't wash off the skin. – By Tom Nebe/dpa